viewerable from metadata should be doc.getRoot().findByGuid(viewableId).parent

[xiaodongliang]

With guid of metadata, we can find viewerable data by doc.getRoot().findByGuid(viewableId), but the viewerable that can be used with loadDocumentNode is its parent.

so the line below should be fixed to

doc.getRoot().findByGuid(viewableId).parent

https://github.com/Autodesk-Forge/learn.forge.viewhubmodels/blob/master/js/ForgeViewer.js#L39-L42

[xiaodongliang]

get parent can only solve the scenario for view in non- BIM 360 Plan folder. To compatible with Plan folder, need to prepare correct version urn and view guid on server side. This PR completes: #17